DE1060372B - Process for purifying crude naphthalene from coal tar in the presence of solid catalysts - Google Patents

Description

Process for purifying crude naphthalene from coal tar in the presence Solid Catalysts The invention relates generally to purification of naphthalene, as obtained from the destructive distillation of coal, in particular such an already partially purified naphthalene, the with this naphthalene occurring impurities at elevated temperatures in Presence of a solid catalyst with the formation of reaction products of im significantly smaller molecular weight are decomposed, while the naphthalene remains essentially unchanged, and the naphthalene thereafter from the reaction products is separated.

Technically pure naphthalene is a naphthalene with a freezing point of 79.60 C, which contains only traces of sulfur compounds, coumarin and no unsaturated Contains hydrocarbons and does not contain phenols or tar bases, a white appearance possesses and remains white for a long time. Usually naphthalene is used this purity for the production of dye intermediates, such as. B. naphthols demands. On the other hand, for a naphthalene, there is a freezing point 780 C there is considerable demand for the oxidation to phthalic anhydride.

Naphthalene is generally used in fractional distillation obtained from coal tar, which may be a by-product of a coking plant can. The nature of the crude naphthalene from coal tar distillation lies between that of an oily semi-solid product whose solidification point is in can be close to 500 C, and that of a more or less crystalline solid Substance whose freezing point can be 770 C. Such crude naphthalene usually contains small amounts of tolunitrile, indene and alkylenes, methyl- and demithyl coumarins, tar acids, such as 2,4- and 2,6-xylenol and ethylene phenols, Tar bases such as Nylidene and Toluidene, some saturated paraffins, polyalkylbenzenes and as the main impurity, thionaphthene.

Crude naphthalene has so far been washed out with sulfuric acid Recrystallization and purified by fractional distillation. To wash of a naphthalene solidifying point 780 C with sulfuric acid for the production of a Naphthalene with a solidification point of 79.60 ° C. has a number of disadvantages in this respect suggest that such washing produces a significant amount of acid sludge, which has a significant proportion of naphthalene, causing a loss of this naphthalene represents, and to the extent that the concentrated levels of those used for this washing Sulfuric acid is expensive and difficult because of its highly corrosive properties are to be handled. Recrystallization has not been satisfactory in that than it requires numerous shallow pans, is tedious and troublesome, and becomes one significant loss of naphthalene results if the procedure is prolonged Time is running. Fractional distillation has not been satisfactory because of the impurities normally found in naphthalene from coal tar are difficult to separate from the naphthalene and usually with the naphthalene during pass the distillation.

It is therefore an object of the invention to provide a new naphthalene purification process to create, which leads to a naphthalene with a high degree of purity and overcomes the aforementioned difficulties of the prior art.

According to the invention, crude naphthalene is obtained from coal tar in the vapor phase in the absence of free oxygen and without the addition of other gases with a catalyst, which is an oxide of a metal from the left column of the Group VI of the Periodic Table on alumina or other suitable support contains, at a temperature of 450 to 8500 C, advantageously from 450 to 6000 C, brought in contact with the naphthalene for a sufficient time to destroy occurring impurities, so that purified naphthalene easily of the reaction products by fractional distillation or cooling or condensation can be separated. Contact times from about 0.01 seconds to 1 minute is satisfactory, but contact times of 1 second to 10 seconds are more desirable. Atmospheric, superatmospheric or subatmospheric pressure can be used at the embodiment 1er invention are applied. However, temperatures are essential below 4500 C not to be used in this process, because those with the naph-: halin Occurring impurities are not sufficiently decomposed at temperatures below 4500 C. werlen. It is also essential that no gas containing free oxygen, such as Air, present in the crude naphthalene vapors to be brought into contact with the catalyst because otherwise a considerable part of the saphthalene will be destroyed.

A refining process for petroleum distillates is known as »Autofining« and their processing products are also known for starting products containing aromatics. In this process, however, a substantial proportion of naphthenes must be used as water nitride supplier be present, while coal tar naphthaine is the only one claimed after the invention Does not contain any noticeable proportions of naphthenes and does not provide for the addition of hydrogen.

When cleaning aromatics from coal tar in the presence of catalysts, which has become known as hygienic "pressure refining", works one with added hydrogen under pressure. This can easily include an undesirable Hydrogenation and decomposition of the end product to be obtained occur what is avoided in the present process.

The new process was commonly used to remove impurities to remove crude naphthalene from a coal tar. But around one at 780 C; melting one Obtaining naphthalene should be a partially purified naphthalene starting product with a freezing point of about 680 C or higher can be used. To a technically Obtaining pure naphthalene or naphthalene that melts at 790 C should be a partial Purified naphthalene with a freezing point of at least 740 C is used will.

The foregoing and other objects and advantages of the invention will emerge even better from the following description together with the drawing, wherein an example of an apparatus for carrying out the invention is illustrated. Of course, the drawing is only used for schematic explanation and not to limit the invention.

According to the drawing, a pump 10 a feeds the crude naphthalene directly from a distillation column or from a storage vessel as long as the naphthalene does not contain a gas containing free oxygen, such as air. through line 10 in the coil 11 of the heat exchanger 12. This impure naphthalene is in the Heat exchanger 12 by indirect Wärmeaustausdi with hot reaction gases, such as will be discussed below, preheated.

When the impure naphthalene to the heat exchanger 12 immediately from is passed to a distillation column. the naphthalene is normally pitch-free.

However, if the impure naphthalene was in stock and as a result has been exposed to air for an appreciable amount of time, it is beneficial to do so Naphthalene undergoing a customary depletion treatment prior to feeding the naphthalene in the heat exchanger 12 to submit. This depreciation treatment includes im generally heating the crude naphthalene in a suitable one, not shown Vessel under superatmospheric pressure to an elevated temperature, for example about 2500 C, Passing the naphthalene to an expansion chamber, not shown, wherein the heated naphthalene is allowed to expand and separate stripping the naphthalene vapors from the top of the chamber and the pitch-like residue from the floor of this chamber.

The heated naphthalene vapors pass through line 13 from the heat exchanger 12 into the coil 14 of another heat exchanger 15, in which the impure naphthalene still further in indirect heat exchange with hot gases from the still to be described Catalyst-lying generation stage is heated, and then go through line 16 on to a three-way valve 17 with valve parts 18, 19 and 20.

When valve parts 18 and 19 are open and valve part 20 is closed, the impure naphthalene vapors go through lines 21 and 22 to reactor 23. If however the valve parts 18 and 19 are closed and valve part 20 is open, go the impure naphthalene vapors through lines 23 a and 25 to the reactor 26. Naturally must be at least valve part 58 or 59 of the three-way valve 24 in the catalyst regeneration gas line also be closed to prevent these vapors from entering the catalytic reactor 26 or reactor 23 pass or that they pass through the catalyst regeneration line going out.

In reactor 23, the impure naphthalene vapors pass through a catalyst layer, which advantageously made of chromium, molybdenum or tungsten oxide on activated There is alumina as a carrier. Mixtures of these metal oxides on alumina can can also be used if desired. The temperature inside the reactor 23 becomes generally to 450 to about 8500 C, advantageously to 450 to about 6500 Cy held. The reactor 23 can be heated by any suitable heating means, so z. B. by a jacket surrounding this reactor through which a suitable Heating fluid is passed, the temperature of which is high enough to the catalyst to bring to the desired reaction temperature.

When the impure naphthalene vapors pass through the catalyst layer in the Go to reactor 23, the impurities associated with the naphthalene decomposed to products of considerably lower molecular weight and thereby easily separable from naphthalene, which remains essentially unchanged. this is surprising. Because during decomposition usually only to a limited extent takes place in a conventional dehydrogenation in which a dehydrogenation catalyst is used, those associated with naphthalene are used in the new process Impurities in smaller fragments or parts in a considerably larger one More decomposed than usual dehydration.

Even impurities containing high molecular weight condensed rings, like quinoline, coumarone and thionaphthene, are completely decomposed while essentially no decomposition of the naphthalene takes place.

Among the fragments isolated from this decomposition are Methane, styrene, toluene and hydrogen sulfide.

The vapors and gases of the reaction products flow from the body of the reactor 23 through lines 24 and 25 to a three-way valve 28 with valve parts 26, 27 and 29. With the valve parts 26 and 27 open and the valve part 29 closed, go the exiting gaseous reaction products through line 30 to the heat exchanger 12 to preheat the incoming impure naphthalene. Then the reaction gases leave by Line 31 to heat exchanger 32 where these hot vapors form an incoming gas preheat for the still to be described regeneration of the catalyst, and go then through line 33 to a three-way valve 34 with valve parts 35, 36, 37th valve 34 can either send the hot reaction products to a fractionation column 39 for separation into a light fraction, a purified naphthalene fraction and a pitch-like fraction Residue or to a cooler 47 and separator 49 for the separation of the liquid Naphthalene from the relatively low molecular weight reaction products, such as hydrogen, Hydrogen sulfide, conduct.

To the gaseous hot reaction products directly to the fractionation column 39 to lead, valve parts 35 and 36 of the three-way valve 34 are opened and valve part 37 closed, so that the gaseous reaction products through line 38 to the middle Part of the fractionation column flow. A light fraction is headed up by 40 removed, which is advantageously provided with a cooler 41. Purified Naphthalene, which is very often already technically pure, is removed through line 42 and advantageously passed through a cooler 43 for condensing the naphthalene. A soil fraction, usually of pitch or pitch-like material, is made up by conduction 44 removed and advantageously passed through a cooler 45. Fractionation column 39 can be fitted with a bubble trap or any suitable filler material be provided.

To the gaseous hot reaction products to a cooler 47 and To conduct separator 49, valve parts 36 and 37 of the three-way valve 34 are opened and valve part 35 closed, so that the gaseous reaction products by conduction 46 to cooler 47 flow. The cooled reaction products then pass through conduit 48 to a separator 49, where the purified liquid naphthalene is separated from the gaseous Separates impurities. The purified naphthalene is withdrawn through line 50, and the gaseous impurities such as hydrogen, methane, hydrogen sulfide, are withdrawn through line 52. The purified naphthalene from line 50 has usually a freezing point above 780 C, but usually it is not technical pure.

This naphthalene can be sold as such or it can be further improved and cleaned by using batch or continuous evaporators evaporated and by means of a suitable fractionation column, e.g. B. Fractionation Column 39, is fractionated.

During the passage of the impure naphthalene vapors through the reactor When the impurities decompose, carbon becomes carbon on the catalyst dejected. Passing the impure naphthalene vapors through the catalyst must therefore be temporarily interrupted if the activity of the catalyst as a result the carbon precipitation decreases in order to regenerate the catalyst.

To this end, the impure naphthalene vapors will pass through interrupted by the reactor.

Then, a regeneration gas containing free oxygen is used Gas, e.g. B. air, or, if desired, water vapor through line 51 into the Snake 52 of the heat exchanger 32 passed and there by indirect heat exchange preheated with hot reaction products from the catalytic reactor, as already described. The heated regeneration gas then goes through line 53 to the coil 54 of the heat exchange 55 to still know ter through indirect heat exchange with hot gaseous products from the catalytic regeneration stage are heated to and then goes through line 56 to a three-way valve 24 with valve parts57, 58 and 59, valve parts S7 and 58 are open, and valve part 59 is closed, so that the preheated regeneration gas through lines 60 and 22 and through the Catalyst layer of the reactor 23 passes therethrough. Since the catalyst is within the Reactor 23 steadily to a temperature within the above temperature range is heated causes the passage of this preheated regeneration gas through the catalyst burning or removing the precipitated carbon from the Catalyst.

The gaseous products of combustion from reactor 23 pass through Lines 24 a and 61 to a three-way valve 62 with valve parts 63, 64, 65th valve part 26 of the three-way valve 28 is closed to prevent this gaseous Products of combustion pass through the conduits for the reaction products. With the valve parts 63 and 64 of the three-way valve 62 open and the valve part closed 65 the gaseous combustion products go through line 66 to the heat exchanger 15, to remove an incoming impure naphthalene fraction, as already described, preheat. After that, these gaseous products of combustion go by conduction 67 to the heat exchanger 55, to the free oxygen-containing gas or the water vapor for catalyst regeneration, and are then passed through line 68 blown off. As soon as the escaping combustion gases the absence of carbon dioxide show, valve parts 57, 58, 63 and 64 can be closed and valve parts 18, 19, 26 and 27 are opened and impure naphthalene vapors are returned to the reactor until this catalyst has to be regenerated again.

When atmospheric air as the gas containing free oxygen An inert gas such as nitrogen is used for catalyst regeneration or carbon dioxide, or water vapor advantageously with the air before regeneration of the catalyst mixed, so as to burn off the precipitated carbon to regulate, since this combustion increases the temperature of the catalyst in the reactor.

Temperatures above about 8500 C can melt and soften the catalyst or otherwise adversely affect them; therefore, excessive climbing should be temperature should be carefully avoided.

In a preferred embodiment of the invention, another, Reactor 26 identical to reactor 23 is provided in a parallel arrangement with reactor 23, so that an impure naphthalene fraction with the catalyst in one reactor can be brought together while the catalyst regenerates in the other reactor is, and vice versa, so an essentially continuous purification of the impure Allow naphthalene fraction. Therefore, while the dirty catalyst in the Reactor 23 by contacting this catalyst with free oxygen containing Gas or water vapor, as previously described, is regenerated, valve parts can 18 and 20 of the three-way valve 17 are opened and valve part 19 is closed, so that preheated impure naphthalene vapors from line 16 through line 23a and 25 are passed to reactor 26 for decomposition of the impurities. When closed Valve part 65 of the three-way valve 62 and open valve parts 29 and 27 of the three-way valve 8th, while valve part 26 is closed, the reaction products go out of reactor 26 through lines 0, 71 and 30 to heat exchangers 12 and 32 and then either to Fractionation column 39 or to cooler 47 and separator 49.

When the catalyst is regenerated after a certain period of operation in the reactor 26 is required. so this becomes in analogously the same Weilse, rie previously bechriehen for the catalyst in the reactor 23 and immediately as shown in the drawing to be taken, carried out. Meanwhile, the impure naphthalene becomes as above described, passed through the catalyst in reactor 23 which has now been regenerated.

Although only two reactors 23 and 26 are shown, it goes without saying possible that a plurality of these reactors are arranged in parallel can with alternating actuation for cleaning; on naphthalene and regeneration. Synchronization of three-way valves can be done electronically by others suitable means are established.

A naphthalene of high purity with a solidification point of 790 C or technical Pure naphthalene can be obtained when the impure naphthalene vapors run out The reactor can be allowed to flow in for t / 2 hours or less. But if that The contact of the catalyst with impure naphthalene vapor takes much longer than 2/2 hour is continued, the amount of leash on the catalyst is reduced Carbon whose activity is so high that the freezing point of naphthalene falls below 790 C. If the catalyst is exposed to impure naphthalene vapors during a is kept in contact for much longer than t / 2 hour, the Regeneration time extended accordingly.

Conventional treatment in the fluidized catalyst bed can be carried out in the reactors 23 and 26 can be used instead of a static catalyst layer.

Should such a moving catalyst bed be used, so the reaction time with impure naphthalene can be longer than that 1/2 hour mentioned, since the catalyst in the fluidized bed is easier can regenerate.

The catalyst used according to the invention consists of a smaller one Proportion of an oxide of an aluminum from the left column of group VI of the periodic Systems, in particular made of chromium, molybdenum or tungsten oxide or a mixture these oxides, on a larger proportion of alumina, are advantageously activated Clay, as a carrier. It advantageously consists of 5 to 25 percent by weight these oxides and from 75 to 95 percent by weight of clay. A preferred catalyst consists of 5 percent by weight chromium. LIolybdäll- or tungsten oxide and 95 percent by weight activated alumina as a carrier and a sufficient amount of an alkali oxide. z. B. Potassium Oxide or Sodium Oxide, based on the weight of this catalyst, about the carbon-forming tendency of this catalyst at the reaction temperature to reduce. It is beneficial to have about 2 percent by weight based on alkali oxide the catalyst weight, to be used.

The catalyst can - as is known per se - by saturating alumina grains or spheres, advantageously made of activated clay. with aqueous solutions a water-soluble compound of a Group VI metal, such as. B. a water-soluble Chromium or molybdenum compound, which if desired also an alkali hydroxide, z. B. Kaliumhvdroxyd, contains appropriate concentration, in Depending on what you want Chromium or molybdenum oxide content and the desired alkali oxide content of the catalyst, getting produced. The resulting catalyst beads are then calcined at 5500.degree. The alkali hydroxide can be added as a solution to the catalyst during its preparation by immersing the prepared catalyst in an alkali hydroxide solution suitable concentration and subsequent heating of the catalyst, for example to 5500 C.

The catalyst can also be obtained by mixing the precipitated oxides Group VI metals such as B. the oxides of chromium and molybdenum, with the precipitated Alumina gel, calcining this mixture at a temperature of about 5500 C and subsequent conversion of the catalyst in the form of spheres, as is known, getting produced.

To make a tungsten oxide catalyst, a hot suspension is used from ammonium tungstate to small spheres or particles of clay, e.g. activated Clay, sprayed, which is then heated to well above 1000 C. After this The coated beads can be sprayed on in the air at an elevated temperature, z. B. be roasted at about 4000 C. for several hours. Instead, you can Finely divided tungsten oxide mechanically with alumina spheres either in wet or blended dry. A mixture of molybdenum, chromium and tungsten oxide The catalyst containing the catalyst can also be produced by this mechanical blending will.

Other suitable methods of preparing the catalyst for the Invention will be known to those skilled in the art.

The following examples are intended to be illustrative but not restrictive thought. Percentages are by weight, unless otherwise stated, and that The naphthalene used comes from coal tar.

Example 1 Vapors of a fraction of distilled naphthalene with The solidification point is 750 ° C. over a catalyst made of 50 / o molybdenum oxide Conducted clay. The temperature on the catalyst surface was 700 to 7500 ° C and the throughput 0.4 volume of liquid naphthalene per volume of catalyst and hour. That had naphthalene purified during the first three hours of operation a freezing point of 78.90 C.

Example 2 Under the same conditions as in Example 1, but with chromium oxide instead of molybdenum oxide as a catalyst. the naphthalene had a freezing point of 78.170 C after 3 hours of operation.

Example 3 A catalyst essentially the same as that in the example 2 used, was with a solution of potassium hydroxide (2 g K OH on 100 g catalyst) treated. The catalyst was then roasted at 5500 ° C. for 2 hours. fumes a fraction of distilled naphthalene with a freezing point of 750 ° C then passed over the catalyst treated in this way, the temperature at the Catalyst surface 700 to 7500 C and the throughput 0.4 volume of liquid naphthalene per volume of catalyst per hour. That gained during 5 hours of operation Naphthalene had a freezing point of 78,450 C.

Example 4 Naphthalene obtained from pans with a freezing point of 77.80 C was passed in the vapor phase over a catalyst, which together precipitated alumina molybdenum oxide (100 / o molybdenum oxide) and with 20/0 K OH, as in Example 2, had been made alkaline. The temperature at the surface of the catalyst was 750 to 800 C and the flow rate was 0.5 volume of liquid naphthalene per volume Catalyst and hour. The naphthalene from the first three hours of operation had a freezing point of 79.230 C. Fractionation of this product gave 75 ovo of a phenol-free naphthalene with a freezing point above 79.60 C.

Example 5 The catalyst from the experiment of Example 4, its Surface was coated with carbon, was treated with a mixture of air and Steam regenerated at the working temperature of Example 4 for 2 hours. This catalyst was then operated for 3 hours with impure naphthalene vapors and using the working conditions of Example 4, a naphthalene was used Freezing point 78.860 C obtained.

Example 6 One by simultaneous precipitation of chromium oxide and alumina produced catalyst (50/0 chromium oxide), alkalized with 2 O / o KOH and at 5500 Roasted C for 20 hours gave a naphthalene with a congealing point of 79.170 C after 3 hours of operation if worked under the conditions of Example 1 became. The naphthalene fed in had a freezing point of 77.80 C.

Example 7 The catalyst from Example 6 was with air at 750 regenerated up to 8000 C for 2 hours. If so, then taking naphthalene vapors the working conditions of Example 6 was brought together, so had in the The naphthalene obtained in the first three hours has a freezing point of 78.970 C.

Example 8 The catalyst from Example 7 was in air at 6000 C heated for 20 hours and then with naphthalene vapors under the working conditions of Example 7 brought together; generated in the first three hours of operation Naphthalene had a freezing point of 79,250 C.

Example 9 Vapors of naphthalene with a congealing point of 680 C were obtained via a simultaneous precipitation of molybdenum oxide-alumina Catalyst (50 / o molybdenum oxide) at 750 to 8000 C and a throughput of 0.3 Volume of liquid naphthalene per volume of catalyst per hour. This in Naphthalene produced in the first two hours of operation had a freezing point from 78,160 C.

Example 10 Naphthalene with a solidification point of 77.80 C was used in the Vapor phase over a catalyst, which is activated from 10 0 / o chromium oxide Alumina consisted of 0.35 volumes of liquid naphthalene per volume throughput Catalyst and hour and passed at a temperature of 725 to 7750 C. That generated Naphthalene had a freezing point of 79.90 C during the first half hour of operation.

Example 11 Vapors of naphthalene with a congealing point of 74.50 C were over a catalyst on activated alumina, impregnated with 5 0 / o tungsten oxide, passed under the working conditions of Example 10. That generated From the first half hour of operation, naphthalene had a freezing point of 79,880 C.

Example 12 Naphthalene with a freezing point of 760 C was in the vapor phase over a catalyst made of activated alumina, impregnated with 20/0 molybdenum, Tungsten and chromium oxide, passed under the working conditions of Example 10. The naphthalene produced had a freezing point from the first half hour of operation from 79.30 C.

Example 13 Distilled naphthalene with a solidification point of 770 ° C. and 3.00 / 0 containing thionaphthene was precipitated from simultaneously over a catalyst Molybdenum oxide-alumina (17 0 / o molybdenum oxide) at a temperature of 450 to 4750 C and a throughput of 1.2 volumes per volume of catalyst per hour. 90 percent by weight of the material fed was with a freezing point of 78.80 C and a thionaphthene content of 1.80 / o during the first half hour of operation won.

Example 14 The same naphthalene as in Example 13 was with a Throughput of 2.4 volumes per volume of catalyst per hour over the same catalyst passed at a temperature of 6100 C. The product of the first 10 minutes of operation gave a yield of 77.2o with a solidification point of 79.5 ° C. and a thionaphthene content of 0.7. In the following 10 minutes, 87.2% was obtained with a time of setting of 78.50 C and 2.3 0 / o thionaphthene. During the third 10 minutes of operation the recovery was 91.20 / 0, congealing point 78.20 C and thionaphthene content 2.8 o / o.

Example 15 It was carried out under the same working conditions as in Example 14 with the addition of 5 volumes per volume of naphthalene vapor per minute worked in air. The recovery during the first 10 minutes was 75%, Freezing point 78.90 C and thionaphthene content 2.4% and in the next 10 minutes 780/0, freezing point 78.50 C, thionaphthene content 2.8 0 / o.

Example 16 Using the same operating conditions as in Example 14, the catalyst was previously treated with preheated hydrogen at a rate of 5 volumes by volume of catalyst per minute for 10 minutes prior to the introduction of the naphthalene vapors. The yields, freezing points and thionaphthene levels during the first three consecutive 10 minute periods are given below. Minutes 0 to 10 110 to 20 1 20 to 30 Yield ............... 84.9 87.9 92.5 Freezing point ...... 79.2 78.5 78.2 Thionaphthene content, ° / o. . 0.49 2.3 2.9 Comparison of Examples 14, 15 and 16 reveals another novel feature of the invention, namely that the purity and yield of naphthalene is improved when the oxidic catalyst of the invention is pretreated with hydrogen or partially reduced.

PATENT CLAIMS 1. Process for cleaning from coal empty obtained crude naphthalene in the presence of solid catalysts, characterized in that that crude naphthalene in vapor form in the absence of free oxygen and without additives other gases with a catalyst which is an oxide of a metal in the left column of Group VI of the Periodic Table contains alumina as a carrier, at a Temperature from 450 to about 8500 C during one for splitting and decomposition sufficient contact time is brought together while the contaminants while the naphthalene remains essentially unchanged, and that the purified one Naphthalene is obtained from the reaction products.

Claims (1)

2. The method according to claim 1, characterized in that reaction temperatures from 450 to 6000 C can be used. 3. The method according to claim 1 and 2, characterized in that the Catalyst from 5 to 25% of the oxide of a metal from the left column of the group VI of the Periodic Table consists of 95 to 75% of alumina as the carrier. 4. The method according to claim 1 to 3, characterized in that a Contact time of 0.01 seconds to 1 minute, preferably 1 to 10 seconds, adhered to will. 5. The method according to claim 1 to 4, characterized in that the Catalyst contains an alkali oxide to reduce carbon deposition. 6. The method according to claim 5, characterized in that the alkali oxide Is potassium oxide. 7. The method according to claim 1 to 6, characterized in that the Bringing naphthalene into contact with the catalyst continues until activity of the catalyst changes significantly as a result of the deposition of carbon on it has reduced that then switched off the catalyst from the naphthalene feed and regenerating that process with the regenerated catalyst as before is continued and from the mixture of the reaction products by fractionated Purified naphthalene is separated by distillation. 8. The method according to claim 1 to 7, characterized in that the Catalyst is pretreated with hydrogen at 450 to 8500 C. Publications considered: German Patent No. 809 812; British Patent Nos. 663 038, 669 553, 692 098, 706 493; U.S. Patent No. 2,739,132.